The subject invention pertains to solid state powder lasers and nanophosphors, to processes for producing powders suitable for such use, and to the powders produced thereby.
Past attempts to utilize highly scattering solid materials as candidates for stimulated emission have required irradiation with high intensity energy sources, such as lasers, to demonstrate stimulated emission. xe2x80x9cLaser paintsxe2x80x9d, surfaces coated with solid particulates, have required such a high threshold level of optical energy input, because of the excessive attenuation (and therefore loss) that normally accompanies scattering, that their suitability for most practical purposes is highly questionable.
Although electron beams are known to cause emission in solid, dielectric phosphors, these materials are not known to support continuous-wave laser action. Atoms in the solid lattice that absorb the electron energy subsequently release it spontaneously and randomly. A large portion of the energy emitted is reabsorbed by the particulate, by its neighbors, or is highly scattered. Phosphors used in CRT tubes, for example, absorb electron beam energy and display spontaneous but not stimulated luminescence, scattering, etc.
Since the discovery of solid state lasers in the early 1960s, lasers have become progressively more commonplace in society, finding applications ranging from compact disc systems and supermarket scanners to precision surgical, optimetric and cutting instruments. The available types of lasers are very diversified, and now comprise solid, liquid, gas and plasma media pumped by light, electrons, chemical reactions, or other means. Generally, lasers require an external cavity to operate.
Luminescence in the multiple-scattering regime has been reported by several researchers from powders containing rare earth and transition metal ions, and extensively investigated theoretically. B. M. Tissue, xe2x80x9cSynthesis and luminescence of lanthanide ions in nanoscale insulating hosts,xe2x80x9d Chem Mater. 10, 2837-45 (1988), and laser action, N. M. Lawandy, R. M. Balachandran, A. S. L. Gomes and E. Sauvain, xe2x80x9cLaser action in strongly scattering media,xe2x80x9d Nature 368, 436-438 (1994); C. Gouedard, D. Husson, C. Sauteret, F. Auzel, and A. Migus, xe2x80x9cGeneration of spatially incoherent short pulses in laser-pumped neodymium stoichiometric crystals and powders,xe2x80x9d J.O.S.A. B10, 2358-2363 (1993); D. Wiersma and A. Lagendijk, xe2x80x9cLight diffusion with gain and random lasers,xe2x80x9d Phys. Rev. E54, 4256-4265 (1996); V. S. Letokhov, xe2x80x9cGeneration of light by a scattering medium with negative resonance absorption,xe2x80x9d Sov. Phys. JETP 26, 835 (1968); S. John and G. Pang, xe2x80x9cTheory of lasing in a multiple-scattering medium,xe2x80x9d Phys. Rev. A54, 3642-3652 (1996). Spectral narrowing and threshold behavior have been reported in the absence of external cavities, and measured transient behavior shown to be characteristic of inverted systems of impurity ions with optical feedback mediated entirely by scattering. Lossless powders in which gain is encountered despite very short scattering mean free path lengths are sometimes referred to as xe2x80x9claser paints.xe2x80x9d D. Wiersma and A. Lagendijk, xe2x80x9cLaser action in very white paint,xe2x80x9d Physics World, 33-37, January 1997.
Laser paint media in which the mean transport length l* is actually less than the wavelength itself have interesting properties which may be useful for speckle-free lithography at sub-micron dimensions or applications in which bright, omni-directional output is desired for displays or light sources of arbitrary shape. However, highly-scattering powders are difficult to pump and study optically because the very scattering that provides the feedback for laser action causes pump light to be scattered backwards very efficiently as it enters the medium. Incident light does not penetrate the medium well and the overall efficiency of any pumping and lasing processes is destined to be low. This effect would be particularly true under conditions of xe2x80x9cstrong localization,xe2x80x9d when light propagation undergoes an Anderson transition, P. W. Anderson, xe2x80x9cAbsence of diffusion in certain random lattices,xe2x80x9d Phys. Rev. 109, 1492 (1958); P. W. Anderson, xe2x80x9cThe question of classical localization: a theory of white paint?,xe2x80x9d Philos. Mag. B52, 505 (1985), to a regime of recurrent scattering which results in completely localized electromagnetic xe2x80x9ctransport.xe2x80x9d D. S. Wiersma, M. P. van Albada, B. A. van Tiggelen, and A. Lagendijk xe2x80x9cExperimental evidence for recurrent multiple scattering events of light in disordered media,xe2x80x9d Phys. Rev. Lett. 74, 4193-4196 (1995). In this regime, scattering would be so strong that the direction of light would be randomized before it has propagated a distance of even one wavelength.
Previous experiments, and theory, on localization of light and stimulated emission in scattering media, as well as interest in the consequences of recurrent scattering events have all heightened current interest in electromagnetic phenomena in multiple-scattering media. See, e.g., M. P. Van Albada and A. Lagendijk, Phys. Rev. Lett. 55, 2692 (1985); V. M. Markushev, V. F. Zolin, and Ch. M. Briskina, Sov. J. Qu. El. 16, 281 (1986); A. Z. Genack and N. Garcia, Phays. Rev. Lett. 66, 2064 (1991); J. X. Zhu, D. J. Pine, and D. A. Weitz, Phys. Rev. A44, 3948-3959 (1991); C. Gouedard, D. Husson, C. Sauteret, F. Auzel, and A. Migus, J.O.S.A. B10, 2358-2363 (1993); N. M. Lawandy, R. M. Balachandran, A. S. L. Gomes and E. Sauvain, Nature 368 436-438 (1994); M. Siddique, R. R. Alfano, G. A. Berger, M. Kempe, and A. Z. Genack, Opt. Lett. 21, 450 (1996); M. A. Noginov, N. E. Noginov, H. J. Caulfield, P. Venkateswarlu, T. Thompson, M. Mahdi, and V. Ostroumov, J.O.S.A. B13, 2024 (1996); D. Wiersma and A. Lagendijk, Physics World, 33-37, January 1997; S. John, Phys. Rev. Lett. 53 2169 (1984); P. W. Anderson, Phil. Mag. B52, 505 (1985); V. S. Letokhov, Sov. Phys. JETP 26, 835. (1968); S. John and G. Pang, Phys. Rev. A54, 3642-3652 (1996); R. M. Balachandran, N. M. Lawandy, and J. A. Moon, Opt. Lett. 22, 319 (1997); D. Wiersma and A. Lagendijk, Phys. Rev., E54, 4256-4265 (1996); G. A. Berger, M. Kempe, and A. Z. Genack, Rev. E56, 6118 (1997); D. S. Wiersma, M. P. van Albada, B. A. van Tiggelen, and A. Lagendijk, Phys. Rev. Lett. 74, 4193-4196 (1995); 18. D. S. Wiersma, P. Bartolini, A. Lagendijk and R. Rhigini, Nature 390, 671 (1997). Traditionally, multiple scattering has been of interest to researchers studying statistical aspects of weakly localized light coherence or imaging. See e.g., M. Kaveh, M. Rosenbluh and I. Freund, Nature 326, 778 (1987), G. Gbur and E. Wolf, Opt. Lett. 24, 10 (1999); E. Wolf, T. Shirai, G. Agarwal, L. Mandel, Opt. Lett. 24, 367 (1999); E. Leith, C. Chen, H. Chen, Y. Chen, D. Dilworth, J. Lopez, J. Rudd, P.-C. Sun, J. Valdmanis, and G. Vossler, J.O.S.A. A9, 1148 (1992). Others have demonstrated powder lasers in the diffusive propagation regime. See, V. M. Markushev, V. F. Zolin, and Ch. M. Briskina, Sov. J. Qu. El. 16, 281 (1986); C. Gouedard, D. Husson, C. Sauterei, F. Auzel, and A. Migus, J.O.S.A. B10, 2358-2363 (1993); M. A. Noginov, N. E. Noginov, H. J. Caulfield, P. Venkateswarlu, T. Thompson, M. Mahdi, and V. Ostroumov, J.O.S.A. B13, 2024 (1996). However, at the boundary between the diffusive and strong scattering regimes it is expected that significant changes will occur in the interaction of light with matter which are fundamentally new. Severe scattering is predicted to cause strong Anderson localization of light when absorption is negligible. Three-dimensional confinement of light within regions of sub-wavelength dimensions could have profound implications for the degree of coherence and back-scattered intensity of elastically-scattered light. The dielectric constant becomes difficult to define when propagation is restricted to sub-wavelength xe2x80x9ctransportxe2x80x9d distances (l* less than 1) in random media, because fluctuations in the structure of the medium and non-uniformities in the field amplitude occur on the same distance scale. See, S. John, Phys. Rev. Lett. 53, 2169 (1984); 11. P. W. Anderson, Phil. Mac. B52, 505 (1985); R. W. Boyd and J. E. Sipe, J.O.S.A. B11, 297 (1994). Additionally, it has been suggested that the onset of recurrent (closed loop) scattering events in this regime will reduce the threshold for laser action mediated purely by scattering feedback. S. John and G. Pang, Phys. Rev. A54, 3642-3652 (1996); and G. A. Berger, M. Kempe, and A. Z. Genack, Rev. E56, 6118 (1997).
The present invention pertains to solid state lasers composed of metal oxide and mixed-metal oxide nanosize phosphor powders, which are preferably doped with one or more transition metals, lanthanide metals, or actinide metals. The present invention also pertains to objects composed of such nanosize particles, which are capable of lasing when stimulated by a suitable stimulation method, particularly by impingement of electrons, including a current or beam of electrons. The objects are preferably in the form of a thin film, or waveguide, and may be suitable for use as a light source, as a flat panel display component, or for other purposes where emission of light by stimulated emission would be useful.
The subject invention further pertains to a process for emitting electromagnetic radiation by stimulated emission, this process comprising assembling a plurality of doped metal oxide nanoparticles, and exposing the nanoparticles to a suitable excitation source, whereupon stimulated emission of electromagnetic radiation results. The source may be a particle beam or current of charged particles, a source of radiant energy such as a laser or flashlamp, or a chemical or explosive reaction. The excitation energy source preferably comprises energetic electrons:
The subject invention further pertains to a process for emitting white light by stimulating an assembly of doped nanoparticles which emit red, blue, and green or other combinations of primary colors which may be viewed as substantially xe2x80x9cwhitexe2x80x9d light. This emission may comprise the joint emissions of several differently-emitting dopant ions in nanoparticles, (mixed within each nanoparticle or residing in separate nanoparticles of a mixed powder), or may be the emission of a single ion species undergoing stimulated emission on more than one transition simultaneously.
The subject invention further pertains to the preparation of highly doped nanoparticles whose dopant concentration exceeds the thermodynamically stable concentration limit, by flame pyrolyzing a ceramic precursor solution containing a dopant concentration in excess of the thermodynamically stable dopant concentration in the solid ceramic.